1. Technical Field of the Invention
The present invention relates in general to an engine system which uses a large size diesel engine as the main power source for driving a vehicle such as a truck, bus etc, and also uses a small size spark ignition engine as an auxiliary power source for driving a supercharger for the diesel engine. This invention especially relates to such an engine system wherein exhaust gas from the spark ignition engine is delivered to the diesel engine where it is treated as exhaust gas recirculation (hereinafter called EGR), and is drawn into the diesel engine for further combustion.
2. Prior Art
Generally, when dealing with exhaust gas generated by the operation of diesel engines, it is known that NOx in the exhaust gas can be decreased if EGR is used. However, although many suggestions have been offered, under present circumstances, most of them have not been realized for practical use because of the following reasons.
First of all, the light oil which is a main fuel for diesel engines contains sulfur(S) which will produce SO.sub.3 during combustion and H.sub.2 SO.sub.4 by combination with moisture in the combustion gas, and these will wear and corrode cylinder walls and other engine parts. However, measures to solve this problem are still incomplete.
Another problem is that ashes in the exhaust gas of a diesel engine are drawn into the engine by EGR and work their way between cams and tappets or piston rings and cylinder walls, producing increased abrasion in these areas. However, this problem doesn't have an effective solution.
Still another problem arises from the fact that it is desirable to cool the exhaust gas by an exhaust cooler during EGR, for such cooling is effective to produce a good EGR result by decreasing the combustion temperature. At the same time, however, this cooling promotes the production of H.sub.2 SO.sub.4 in the exhaust gas, resulting in corrosion of the exhaust cooler, and also possibly increases wear and corrosion of the cylinder wall and other engine parts by cooled H.sub.2 SO.sub.4. However, effective measures for reducing wear and corrosion by H.sub.2 SO.sub.4 have not been found yet, as stated above.
With regard to NOx in the exhaust gas of the diesel engine, although remaining exhaust NOx is decreased to some degree by partial exhaust EGR as mentioned above, some NOx still remains in the discharged exhaust, and this NOx in the exhaust gas cannot be further decreased under the present circumstances.
A catalyst might be used to decrease NOx in an spark ignition engine; however, because the exhaust gas from a diesel engine contains oxygen, a catalyst which can remove NOx effectively and economically has not been introduced yet.
A further problem is that although the increase of maximum combustion pressure resulting from supercharging a diesel engine improves the mechanical load of the engine, it also produces an increase in NOx generation as a result of an increase in combustion temperature. If the compression ratio is decreased to solve the NOx problem, then engine starting becomes difficult, and generation of harmful white smoke immediately after the start will result. With the above situation, although a decrease of compression ratio to 11 or 14 can be effective, it has not succeeded under the present condition.
Research is continuing in an effort to design combustion chambers and combustion methods and to find catalysts which will effectively remove NOx from exhaust gases and prevent the corrosion and abrasion described above. However, this matter is not solved yet.
A still further problem with diesel engines, and the biggest problem when conducting EGR, is that the diesel engine output decreases with EGR.
On the other hand, as is generally known, the spark ignition engine, which uses fuels such as gasoline, propane gas, natural gas etc., doesn't contain sulfur, so that special measures to prevent wear and corrosion by H.sub.2 SO.sub.4, as mentioned above,are not required. Furthermore, in addition to the use of EGR, NOx can be removed, along with other exhaust pollutants (CO, HC), by catalytic converter rhodium (CCRO). Therefore, the exhaust gas from an spark ignition engine contains almost no NOx.